sustainability Article Assessment of State Transition Dynamics of Coastal Wetlands in Northern Venice Lagoon, Italy Andrea Taramelli 1,2 , Emiliana Valentini 2,3, Laura Piedelobo 1,* , Margherita Righini 1 and Sergio Cappucci 4 1 Istituto Universitario di Studi Superiori di Pavia (IUSS), Palazzo del Broletto, Piazza della Vittoria 15, 27100 Pavia, Italy; [email protected] or [email protected] (A.T.); [email protected] (M.R.) 2 Institute for Environmental Protection and Research (ISPRA), via Vitaliano Brancati 48, 00144 Rome, Italy; [email protected] 3 Institute of Polar Sciences of the Italian National Research Council (ISP CNR), via Salaria km 29, 300-00015 Rome, Italy 4 Territorial and Production Systems Sustainability Department, Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), via Anguillarese 301, 00123 Rome, Italy; [email protected] * Correspondence: [email protected] Abstract: Coastal wetlands represent particularly valuable natural resources, characterized by the interaction between their geomorphological and biological components. Their adaptation to the changing conditions depends on the rate and extent of spatial and temporal processes and their response is still not fully understood. This work aims at detecting and improving the understanding of the transition dynamics on eco-geomorphological structures in a coastal wetland ecosystem. The approach could support sustainable habitat management improving the detection and optimizing the offer of Earth Observation (EO) products for coastal system monitoring. Such course of action will Citation: Taramelli, A.; Valentini, E.; strengthen evidence-based policy making, surface biophysical data sovereignty and the Space Data Piedelobo, L.; Righini, M.; Cappucci, downstream sector through remote sensing techniques thanks to the capability of investigating larger S. Assessment of State Transition scale and short-to-long-term dynamics. The selected case study is the Lido basin (Venice Lagoon, Dynamics of Coastal Wetlands in Italy). Our methodology offers a support in the framework of nature-based solutions, allowing Northern Venice Lagoon, Italy. the identification of ecosystem-level indicators of the surface biophysical properties influencing Sustainability 2021, 13, 4102. https:// stability and evolution of intertidal flats on which a conceptual model is implemented. Landsat doi.org/10.3390/su13084102 satellite imagery is used to delineate the spatial and temporal variability of the main vegetation and Academic Editors: Konstantinos sediment typologies in 1990–2011. Within this period, specific anthropic activities were carried out Stefanidis and Elias Dimitriou for morphological restoration and flood protection interventions. Specifically, the lower saltmarsh shows its more fragmented part in the Baccan islet, a residual sandy spit in front of the Lido inlet. The Received: 11 March 2021 area covered by Sarcocornia-Limonium, that triggers sediment deposition, has fluctuated yearly, from a Accepted: 1 April 2021 minimum coverage of 13% to a maximum of 50%. The second decade (2001–2009) is identified as the Published: 7 April 2021 period with major changes of halophytic and Algae-Biofilm cover typologies distribution. The power law and related thresholds, representing the patch size frequency distribution, is an indicator of the Publisher’s Note: MDPI stays neutral ecosystem state transition dynamics. The approach, based on multi-temporal and spatial EO analysis, with regard to jurisdictional claims in is scalable elsewhere, from regional to local-to-global scale, considering the variability of climate data published maps and institutional affil- and anthropogenic activities. The present research also supports sustainable habitat management, iations. improving the detection, and optimizing the offer of EO products for coastal system monitoring. Keywords: coastal wetland; bio-geomorphology; Venice Lagoon; satellite remote sensing; Landsat; Linear Spectral Mixture Analysis; Empirical Orthogonal Function; Power Law Distribution; coastal Copyright: © 2021 by the authors. zone management Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons 1. Introduction Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Coastal wetlands represent complex ecosystems prone to continuous changes. These 4.0/). changes are especially standing out nowadays due to the existing feedback loop between Sustainability 2021, 13, 4102. https://doi.org/10.3390/su13084102 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 4102 2 of 24 the impact of climate change and anthropic activities, ecosystem degradation, and the increased risk of natural disaster. The morphological change of land processes can be fully understood as long as all the significant interactions between the physical and biological components are considered [1–6]. The role of the eco-geomorphological responses caused by the interaction between biotic and abiotic processes (e.g., vegetation growth, sedimentary processes, microphytobenthos influences, waves, tides or storm surges), has been recently recognized [7–13], leading to the development of new approaches. Special emphasis is given to bio-geophysical processes when studying the change of intertidal landscapes [3,4,14]. Intertidal mudflats are fine sediment deposits that are periodically submerged during the high tide. Mudflats have a worldwide distribution. They are ephemeral on a geological time-scale (they last a few thousand years), but have a fundamental ecological role and are characterized by extremely high biological productivity, which plays an important role in the balance between marine and terrestrial ecosystems [15,16]. Due to intermittent flooding and exposure to the atmosphere, they are characterized by many different physical and biological processes that can influence their composition and habitats [17–19]. Generally, they are composed of coarser sediment in the subtidal areas (below the low tide level) and finer sediments in the upper part (between the mid and the high tide levels). Mudflats are classified according to the tidal range, wave energy, sediment supply, morphology, physical and biological properties [20]. The tidal range is one of the most important parameters for the classification because of its role in controlling bed morphology. However, the sedimentological, biological, and oceanographical characteristics of mudflats still need to be deeply understood [21]. Lagoons’ ecosystems are represented by low-elevation bare flats and upper vegetated saltmarshes. These components are constantly interacting [22] and are thereby prone to continuous changes of their internal equilibrium. Vegetation’s spatial distribution stands out for its complex patterns and striking features. Particularly in saltmarshes, given the self-organization in patchy distributions [13,23–28], a key role in ecological stability and disturbance-recovery processes has been observed [29–32]. In spatially organized ecosystems, the patchiness can be generated by a wide variety of dynamics. Identifying the way vegetation, substrates and waters are distributed and organized, as well as the manner of change under the influence of the surrounding environment, provides valuable information to deduce the system’s behavior and predict its spatial dynamics. On the other hand, eco-geomorphological dynamics of saltmarshes are strongly influenced by the wide range of spatial scales of the changing processes, from few centimeters to several kilometers. External forces cannot explain solely the status of ecosystems but can influence the spatial bio-geomorphological structure organization. Accordingly, ecosystem- or landscape- level indicators are being continuously developed as a manageable set of indicators that reflects the structural and functional properties of the landscape units (i.e., effects of anthropogenic and environmental disturbances) [33]. For instance, the self-organized patchiness of sediments and vegetation is often used as an indication of ecosystem stability and dynamics [28]. The way in which spatial patterns reflects the level of disturbances exerted on the ecosystem and thereby its level of stability can be used to understand whether self- organization induces stability in ecosystems or pushes ecosystems towards the edge of collapse [29,34–36]. The elevation of sediment banks from subtidal to intertidal levels is the first step to trigger bio-stabilization and colonization of pioneering vegetation. The competent authorities started to adopt more practical but scientific-based solutions, demon- strating the important role of mudflat morphology (elevation and time of exposure) on bio-stabilization processes and its effect on the resilience and development of intertidal areas within the Venice Lagoon [18,19,37]. The advantage of reducing mudflats erosion, other than habitat conservation, is to lower sedimentation in the natural channels, and remote sensing provides a defense model for detecting sediment erosion. Nowadays, the opportunities offered by the synergies Sustainability 2021, 13, 4102 3 of 24 between traditional and innovative remote sensing platforms (e.g., satellites and drones) allow the near real-time monitoring of natural and human-induced changes in tidal flows. The blended monitoring approach (traditional with innovative) allows expanding the model to a variety